The invention relates generally to an active suspension system for an automobile, and more particularly to stabilizing an electric active suspension upon interruption of the power supply to the suspension during key off.
Various types of active suspension systems have been proposed which are capable of responding to road inputs in real time with power inputs having sufficient robustness to control body motion of an automotive vehicle. As used herein, the term "active suspension" means a suspension which has a frequency response and power output characteristics sufficient to respond in real time to control force inputs to the vehicle chassis arising from such sources as road imperfections, crosswinds, and vehicle turning maneuvers. As used herein, the terms "body" and "chassis" mean either a conventional unitized automotive body or conventional frame and body automobile or any combination thereof. Active suspensions can be hydraulic systems wherein a hydraulic actuator in conjunction with complex valve componentry is used to maintain the ride and handling characteristics of the vehicle as well as the vehicle height at its optimum considerations. Other types of active suspension systems are electrically powered systems such as those disclosed in U.S. Pat. Nos. 5,027,048; 5,060,959 both assigned to the assignee of the present invention, as well as 5,028,073; 4,969,662; and 4,892,328.
Active suspensions are particularly suited for use in high profile vehicles such as sport/utility vehicles, pickup trucks or vans. A feature that can be provided for these vehicles is to lower the suspension below the designed ride height to a jounce position to facilitate entry and exit to and from the vehicle. One known method of accomplishing this result is to bleed off air from an air suspension system. One drawback of such a system is that it takes a relatively large amount of time to lower the suspension down to a desired level. Another disadvantage is that the air suspension also needs a considerable amount of time to increase the air volume in the system to raise the suspension back to the design height.
One proposed concept for a fail-safe system is disclosed in commonly owned U.S. Pat. No. 5,070,284. The fail-safe system described therein places a resistance in the circuit should the suspension fail while the vehicle is in operation. When the vehicle is in operation, the vehicle suspension is powered by the engine alternator. The system provides a known load resistance to force the suspension to simulate an actual shock absorber. This system, however, cannot be used if the vehicle is not in operation or the power source fails. If the vehicle was, for example, in the key off position with the vehicle in a jounce or extended jounce position and the power source is interrupted, the electric suspension releases and returns to its design height at a rate controlled only by the spring constant and the vehicle mass. This rate would generally be uncomfortable for a passenger.
It is therefore desirable to restrain the rate of release of an electric active suspension should an interruption in a power source occur when the vehicle is in a jounce position.